Skeletal isomerization of 1-butene to isobuten over acid catalysts obtained by grafting silicon alkoxide on γ-alumina

A study was made of the preparation and properties of some acid catalysts obtained by grafting silicon alkoxides on γ-alumina. All the prepared catalysts were tested in the skeletal isomerization of 1-butene to isobutene. Isobutene is an intermediate in the production of MTBE, today the most important oxygenated compound used as an additive in lead-free gasoline. The discussion covers introduction; experimental (catalyst preparation and characterization); results (determination of the catalyst surface properties by potentiometric titrations and TGA analyses; TPD determination of acid surface properties; XPS determination of surface properties of silicated alumina catalysts; and catalytic activities and selectivities in skeletal isomerization of 1-butene to isobutene). The type of acidity, Lewis or Brönsted, and its strength affects selectivity, and by examining published data, the best catalysts seemed to be those with acid sites of medium strength because the selectivity toward isobutene is enhanced and catalyst deactivation prevented. Silicate aluminas have these requisites. Therefore, the acidities of the catalysts A1S1, A1S2, A1S3, A1S4, and A1SM were tested and, for comparison, those of silica, silica-alumina, and alumina support. The highest conversion and selectivity to isobutene was over A1S4. The conversion was 83.3% and the products distribution was isobutane 0.69, n-butane 0.18, 2-trans-butene 23.7, isobutene 36.7, 2-cis-butene 17.5, and 2-cis/2-trans butene 0.74. The catalysts A1S4 and A1SM also showed a remarkable stability.